The invention relates generally to material testing and more specifically concerns bearing-bypass testing of mechanically-fastened joints in materials such as composites.
In the past, design procedures for mechanically-fastened joints in composites have usually been very conservative. In such cases, failures were usually avoided by heavily reinforcing the laminates in the vicinity of the joints. Needed improvements in joint efficiency require data bases for laminates tested under conditions typical of structural joints. Within a multi-fastener structural joint, fastener holes may be subjected to the combined effects of bearing loads and loads that bypass the hole. The ratio of bearing load to bypass load depends on the joint stiffness and configuration. As the joint is loaded, this bearing-bypass ratio remains nearly constant until damage develops. Although the combined effects of bearing and bypass loads can be simulated by testing single-fastener specimens, such tests are difficult.
Three approaches to bearing-bypass testing have been used in the past with simple specimens. The first approach uses levers and linkages to divide the applied load into two proportional parts. One part acts on the end of the specimen and the other is reacted as a bearing load at the specimen hole. The bolt hole is thereby subjected to proportional bearing and bypass loading. The lever fulcrum points can be changed to produce different ratios of bearing to bypass loading. This lever-linkage approach works well for tension bearing-bypass loading, but is difficult to apply in compression.
The second approach to bearing-bypass testing uses a "scissor" mechanism to apply a bearing load between two holes in the test specimen. This bearing load is held constant while the bypass load is increased until the specimen fails. Although this approach does produce bearing-bypass loading in tension or compression, it does not maintain the desired constant ratio of bearing to bypass loads. Furthermore, this approach could alter the sequence of local damage development.
The third approach used two control systems: one controls the bearing load while the other controls the bypass load. The bypass load is applied to one end of the specimen in the conventional manner; however, the bearing load is applied through linkages with two hydraulic cylinders connected to the ends of a bearing bar which is bolted to the specimen. Although this concept works in tension and compression the test apparatus is rather complex and quite different apparatus arrangements are needed by the two types of loading.
An object of this invention is to provide bearing-bypass testing approach with a constant bearing-bypass load ratio throughout.
Another object of this invention is to provide a simple approach for combined bearing-bypass testing that works equally well in tension and compression.
Another object of this invention is to provide bearing-bypass testing that requires no change in apparatus in tension and compression testing.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawing.